April 2010

Multiple factors affect running shoe selection

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The concept of evidence-based running shoe selection is nice in theory, but may be easier said than done. In fact, the literature is replete with conflicting findings on a range of issues, from arch height to shoe construction.

Distance running is extremely popular due to its cardiovascular fitness benefits and minimal equipment requirements. Between 8 and 16 million Americans identified themselves as “frequent” runners (running more than 100 days annually) in 2007.1 Unfortunately, many runners sustain one or more overuse injuries over time. Following a recent literature review, Van Gent and colleagues reported that the incidence of running-related injuries ranges from 19 to 79%.2 As many as 37% to 56% of all runners sustain at least one injury annually; between 20% and 70% of these individuals seek medical treatment for their injuries.3.4

The severity of running-related injuries can significantly impact both exercise and occupational activities. Between 30 and 90% of running-related injuries cause runners to decrease or halt their training efforts.3 Lun reported that injuries sustained within a group of 87 recreational runners most commonly caused male runners to decrease running mileage for more than seven days and female runners to stop running for more than seven days.5 Meanwhile, as many as 5% percent of running-related injuries may result in lost work time.3 Further, Smith and colleagues have reported that running-related injuries among U.S. Army light infantry soldiers result in seven times more limited duty days than non-running injuries.6

Running injuries have been associated with altered foot motion or excessive force transmission due to repetitive foot contact with the ground.7-9 Researchers have suggested that running in a shoe specific for one’s foot type can influence rearfoot motion or shock absorption sufficiently to reduce the risk of injury.7 Thus, many clinicians prescribe specific running shoe types based upon individual foot type to minimize risk of injury. The purpose of this article is to provide an overview of specific foot types and associated running shoes, highlight the research to date that addresses the effect of shoe type on running biomechanics and injury risk, and provide an overview of running shoe fitting and replacement guidelines.

Foot types and injury risk

Many clinicians and researchers believe that extremes of foot type increase one’s risk for injury. The prevalence of foot types within the adult population is unclear. Subotnick has reported that 60% of the general population has a normal-arched foot, while low- and high-arched feet each account for 20% of the population.10 The authors have assessed foot type among 1000 male and female military and their adult family members using the Arch Height Index, based on heel-to-toe length.11 Within this group, 69% had a normal arch, while 17% had a low or extremely low arch. The remaining 14% had either a high or extremely high arch. Other estimates of the prevalence of low-arched feet among adults range from 1.8% to 23%, while estimates of the prevalence of high-arched feet range from 8% to 15%.12,13

To our knowledge, few authors have addressed the prevalence of foot types among distance runners. Burns and colleagues assessed static foot type among 131 triathletes with the Foot Posture Index (FPI) and Valgus Index (VI) measures. The prevalence of low-arched feet per each of these measures was 6% and 4%, while the corresponding prevalence estimates for high-arched feet among the triathletes were 9% and 2%.14 Underlying reasons for the varied estimates of foot type within the general adult and athletic populations include the varied methods used to assess arch height and the varied criteria used to categorize a foot as low, normal or high-arched. The prevalence of low-arched feet in the general adult population also appears to vary by race.15

The low-arched or planus foot has been historically linked with increased mobility. The low-arched foot and increased rearfoot eversion or pronation have been associated with increased risk for medial, soft tissue (non-bony) and knee injuries.16-18Williams and colleagues have suggested that increased pronation may increase stresses on the medial lower extremities, whereas increased rearfoot eversion excursion and velocity as demonstrated by the low-arched foot may force the medial foot and ankle to exert greater active and passive control of lower extremity movement.17

However, some researchers have reported that the low-arched foot does not place a runner at increased risk for injury.19-21 In fact, Giladi and colleagues have reported that the low-arched foot has a decreased risk of stress fracture compared to either the normal- or high-arched foot.20 The increased motion associated with the flexible, low-arched foot may actually result in increased shock absorption capability, and may therefore protect against specific types of lower extremity injuries associated with running (e.g., stress fractures).

The high arched or cavus foot tends to be stiffer than either the low or normal arched foot. Researchers and clinicians generally agree that the rigid, high arched foot places one at increased risk due to its lesser ability to absorb impact forces. The rigid, high-arched foot in runners has been associated with increased risk for lateral, bony and ankle injuries.17 However, it has also been suggested that the flexible, high-arched foot/ankle may not be at increased risk for injury.22 Further, Wen and colleagues have reported that the high-arched foot as determined by arch index measurements may even decrease a runner’s risk both for running injuries in general and for knee injuries specifically.23 They did not suggest why the high arched foot might provide a protective effect. A potential explanation for these findings is that a flexible foot, regardless of arch height, may be better able to absorb shock or impact forces.

Running shoe manufacturers have designed specific types of shoes to address these biomechanical differences. The motion control running shoe is generally prescribed for either low arched feet or for normal arched feet with increased foot mobility. The motion control shoe generally has a firm, multidensity medial midsole, medial posting and a rigid heel counter. This shoe is designed to prevent excessive rearfoot pronation, which may result in medial midsole compression and inward shoe tilt such as in Figure 1.4

The cushioned or neutral running shoe is generally prescribed for either high- arched feet or for normal-arched feet with decreased foot mobility. This shoe generally has a soft, single density medial midsole and no medial posting. This shoe is designed to enhance shock absorption. Decreased foot mobility may result in lateral midsole compression and outward shoe tilt such as in Figure 2.4

The stability running shoe is generally prescribed for normal arches. It may also be prescribed for low or high arched feet with normal mobility. This shoe generally has a moderately firm, multidensity medial midsole and medial posting. It is designed to provide a balance of medial support and shock absorption.

Some shoes are marketed as racing/training or lightweight shoes.4 Competitive runners frequently wear these shoes during high intensity training or competitions. A relatively recent addition to the running shoe market is the minimalist shoe. Proponents of the minimalist shoe design promote its ability to simulate barefoot running while protecting the foot. While one recent study provides preliminary support for these properties, significant debate remains within the medical community and running shoe industry concerning the relative benefits of the minimalist shoe.24,25

Effect of shoe type on biomechanics

Manufacturers have designed running shoes to address biomechanical parameters with the goal of decreasing injury risk. One objective of running shoe design has been to control rearfoot motion. Several researchers have reported that running shoes affect rearfoot motion minimally, if at all.26-28However, others have found that running shoes can affect rearfoot motion or midfoot contact areato varying degrees, dependent in part upon the methodology for measuring motion.7,29-32Butler and colleagues have suggested that even a relatively small change in rearfoot motion could be clinically significant given the multiple footstrikes associated with running.7

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Although many clinicians and researchers focus on rearfoot motion, significant movement occurs at the midfoot during gait.33 Further, Murphy and colleagues have reported high correlation between normalized midfoot contact area and plantar pressure values during gait.34 Further research addressing the relative effects of running shoes on midfoot motion may benefit future efforts to reduce running-related injury risk.

A second objective of running shoe design has been to increase shock absorption. Conflicting findings exist concerning the shock absorption capabilities of running shoes. Several researchers have reported that midsole properties can affect lower extremity loading and force transmission.7,35 Others have reported that midsole properties minimally affect the magnitude of external impact forces.36-38

A potential limitation to studying biomechanical changes associated with running shoe wear is the response of the human body to changes in footwear or running surfaces. Researchers have proposed that a runner adapts to such changes through neuromuscular control mechanisms or adjustments to running mechanics.22,36,38,39 This adaptive ability could diminish any potential effect of running shoes on rearfoot motion, force transmission or shock absorption.

A topic for further study is the potential effect of running shoes on rearfoot motion and/or force transmission once a runner has fatigued.7Butler and colleagues have proposed that running shoes may exert a greater effect on running mechanics as the runner progressively fatigues.7 It is believed that most running-related injuries occur when a runner is fatigued, as subsequent decreases in volitional muscle strength may result in increased lower extremity joint motion.7 It is also possible that fatigue increases the time required for a runner to react to, or compensate for changes in the running surface.

Butler and associates reported that motion control shoe wear reduced tibial internal rotation but not rearfoot eversion among low-arched runners during prolonged running.40 They hypothesized that the motion control shoe might have provided midfoot support that influenced tibial rotation but not rearfoot movement. They also found that the low-arched runners’ rearfoot mechanics did not change across the prolonged run. While this might reflect adaptation as discussed above, it is possible that the prolonged run dictated by the study’s protocol was not sufficiently fatiguing in terms of speed or distance to induce rearfoot mechanical changes.40

Butler and colleagues also found that cushioned shoe wear reduced tibial shock among high-arched runners when compared with motion control shoe wear, but that tibial shock did not change significantly across the course of the prolonged run. While the absence of a change in tibial shock might again reflect adaptation, it is once more possible that the prolonged run was not sufficiently fatiguing in terms of speed or distance to induce lower extremity mechanical changes.40

Shoe type and overuse injury risk

Despite the common practice of prescribing shoe type based upon foot type, minimal evidence supports shoe prescription to reduce injury risk.Authors of a recent literature review found no randomized, controlled trials addressing the effect of shoe prescription on injury rates. The authors concluded that running shoe prescription based upon foot type is not evidence-based.41 Further, several researchers have questioned whether increased cushioning or rearfoot pronation control could decrease injury risk.28,42

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Knapik and colleagues recently assessed the effect of running shoe prescription on injury risk among entry-level military trainees.43 Even when limiting their comparison to high and low arched trainees, Knapik and his fellow researchers found that shoe prescription based on static plantar foot shape did not reduce training-related injury risk when compared to providing each trainee with a pair of new stability running shoes (regardless of static foot type).They subsequently recommended that military trainees merely obtain new running shoes of any type upon beginning entry-level training.

What remains unknown is the effect of shoe prescription on injury risk among moderate to high mileage runners. Participants in Knapik’s study performed distance runs of 0.5 to 3.0 miles and/or sprints approximately two to three times weekly.43 This training volume falls below the thresholds associated with increased injury incidence within the general population (running more than three times per week, 30 minutes per session or 40 miles weekly).44,45

Also unknown is the effect of running shoe prescription as a means of secondary injury prevention among currently or previously injured runners. Further unknown is the effect of prescribing running shoes based upon foot mobility rather than upon static foot type alone.7 Thus, it is possible that running shoe prescription based upon foot type might reduce injury risk among runners with higher training volumes or among runners with histories of prior running-related injuries. It is also possible that accounting for foot mobility when categorizing foot type might enable running shoe prescription to reduce injury risk within the general running population. More research is required to determine the effect of running shoe prescription based upon both foot structure and mobility.

Fitting and replacement guidelines

The minimal evidence-based support for running shoe prescription in primary injury prevention has led some to recommend that runners select and replace shoes based solely upon comfort, fit and overall shoe condition. Taunton and colleagues linked older running shoe wear to increased injury risk among women preparing for a 10 km running race.46 Women who ran in shoes that were one to three months old sustained fewer new injuries than women whose shoes were four to six months old. However, in the same study, male runners who wore shoes that were four to six months old sustained fewer injuries than their counterparts who wore newer shoes.

A potential limitation of these findings is the fact that the participants in Taunton’s study were not matched for aerobic fitness level, previous running history/training base or current running mileage. Each of these characteristics affects one’s risk for sustaining running-related injuries; it is possible that Taunton’s male participants with older running shoes were fitter than, or had a greater training base than their counterparts with new shoes. It is also possible that some male participants had recently purchased running shoes in response to a prior running-related injury. If so, new shoes would merely be a response to, rather than a risk factor for sustaining a running-related injury.46

Figure 3. There should be at least half a thumb's width but not more than a full thumb’s width of space between the end of the longest toe and the tip of the shoe.

Gardner and colleagues have also identified a “slight trend” linking running shoe age and stress fracture incidence.47 In Gardner’s study, approximately 1.1% of Marine recruits whose running shoes were less than one month old sustained a stress fracture across the study period. In the same study, approximately 1.9% and 2.5% of trainees whose shoes were between one and six months or six and 12 months old sustained a stress fracture. Interestingly, no stress fractures were sustained by recruits whose running shoes were more than one year old. Gardner did not hypothesize as to why these recruits sustained no stress fractures. However, it is possible that at least some of these recruits were regular runners before beginning entry-level military training, and thus were at lower risk of training-related injuries than their previously sedentary counterparts.

Cook and colleagues reported in 1985 that running shoes lost approximately 45% of their shock absorption capability between 250 and 500 miles of machine-simulated running.48 Shock absorption degradation tapered off beyond this point. By 500 miles of machine-simulated running, the various models tested maintained from 45% to 60% of their initial shock absorption capability. The shoes were subjected to mechanical deformation that simulated repeated heel strikes during running. Cook also assessed shock absorption degradation in shoes that were subjected to actual running conditions. In contrast to the shoes that underwent mechanical deformation, the shoes subjected to actual running maintained approximately 70% of their initial shock absorption capabilities after 500 miles of wear. More recently, Zhang and colleagues reported that running shoes undergo a gradual degradation of cushioning capability between 50 and 400 miles of wear.49

Figure 4. Each side of the shoe should be half a thumb’s width from the edge of the ball of the foot.

In regard to shoe comfort, Nigg has proposed that a comfortable running shoe may reduce a runner’s energy expenditure requirement by minimizing muscle activity that occurs in response to shoe wear.26 Shoe comfort could affect injury risk in two ways. First, a lessened energy expenditure requirement could delay the onset of volitional muscle fatigue, as discussed earlier. Also, an uncomfortable running shoe could cause a runner to adopt a compensatory or abnormal gait pattern that subsequently increases the runner’s risk of injury.

The authors recommend that runners address both shoe length and width when selecting new running shoes. The runner should try on shoes while wearing the same type of sock and any foot orthoses or insoles that he or she typically uses when running. One should check the fit for both feet while standing. There should be at least half a thumb’s width but not more than a full thumb’s width of space between the end of the longest toe and the tip of the shoe (Figure 3). Each side of the shoe should be half a thumb’s width from the edge of the ball of the foot (Figure 4). The shoe should be comfortable from the start; there is no “break in” period associated with running shoes.4

It is frequently recommended that a runner replace his or her shoes every 400 to 600 miles.4 Asplund has further noted that a running shoe midsole’s shock absorption capability naturally degrades after one to two years, even if the shoe has never been worn. For this reason, he recommends that runners replace their shoes at least every six months, regardless of mileage.4

Based upon the available research to date, the U.S. Department of Defense’s Joint Services Physical Training Injury Prevention Work Group has noted that “reports from shoe manufacturers and biomechanical studies on running shoes show that shoes should provide satisfactory support and cushion for 400 to 600 miles of use and, therefore, should be replaced accordingly to prevent injury.”51 However, the Work Group also noted that the evidence to date was insufficient to recommend replacing running shoes at specific mileage intervals to reduce injury risk. Based upon the evidence to date, the authors of this article recommend that a runner replace his or her shoes every 400 to 600 miles. Runners with a history of injuries, those with extremes of arch height/foot mobility, and heavier runners should consider replacing shoes closer to 400 miles of wear. Shoes with evident medial or lateral midsole compression (Figures 1 & 2) require replacement regardless of mileage.

Overview

In light of the conflicting findings and ongoing debate associated with running shoe-related research, clinicians should emphasize the importance of shoe comfort and be ready to provide appropriate shoe sizing and replacement guidelines to their patients. In regard to running shoe type, clinicians must continue to incorporate good clinical judgment, their knowledge of biomechanics, and individual patient history of running-related injuries to help guide each patient in selecting an appropriate shoe. Future research efforts should focus on measures of shock absorption and rearfoot motion control while the runner is fatigued, the impact of running shoe prescription on injury incidence among moderate to high mileage runners, running shoe prescription as a means of secondary injury prevention, and the effect of shoe prescription based upon foot mobility rather than static foot type alone.

Joseph M. Molloy, PT, PhD, SCS, is adjunct faculty for the U.S. Army-Baylor University Doctoral Program in Physical Therapy, and clinical internship director in the Physical Therapy Service at Brooke Army Medical Center, both in Fort Sam Houston, TX. Deydre S. Teyhen, PT, PhD, OCS, is an associate professor and director of the Center for Physical Therapy Research in the U.S. Army-Baylor University Doctoral Program in Physical Therapy.

The opinions or assertions contained in this article are the views of the authors and should not be construed as official or reflecting the views of the United States Army or the Department of Defense.

42. Hennig EM. Plantar pressures, shock and rearfoot motion during running – are these meaningful quantities for the prediction of running injuries? Presented at IV World Congress of Biomechanics, Calgary, August 2002.